The human spine comprises individual vertebrae that interlock with each other to form a spinal column. The vertebral elements of a vertebra are labeled in FIG. 1. The vertebra 100 has a vertebral body 102, two pedicles 104 extending from the vertebral body, a lamina 106 extending from the pedicles, and three winglike projections. These winglike projections are the spinous process 108 extending from the lamina and the two transverse processes 110 extending from the pedicles. In addition, the vertebra 100 has two superior articular processes 112 extending from the pedicles and two inferior articular processes 114 extending from the lamina. The articular processes of adjacent vertebrae form the facet joints. The inferior articular process of one vertebra articulates with the superior articular process of the vertebra below. Adjacent vertebrae are separated and cushioned by an intervertebral disc 116.
Together, two adjacent vertebrae, their facet joints, the intervertebral disc and the connecting ligament tissue make up a motion segment unit, the smallest portion of the spine that exhibits kinematic behavior characteristic of the entire spine. The motion segment unit is capable of flexion, extension, rotation, lateral bending and translation and each component of the unit contributes to the mechanical stability of the entire unit. Trauma, degeneration, aging, disease, surgery, and the like may damage any of the components of the motion segment unit, leading to instability in the unit and causing severe pain, numbness, decreased mobility, muscle weakness and nerve damage to the patient.
One approach to treating these spinal conditions involves spinal fusion. In spinal fusion, two or more adjacent vertebrae are permanently fused by forming a bony bridge between the vertebrae in order to stabilize and immobilize the motion segment unit. Ligaments, bone, disc, or combinations thereof may be removed prior to fusion. Hardware in the form of bars, plates, rods, screws and cages may be used in combination with bone graft material to facilitate stabilization and fusion of the vertebrae. By placing the adjacent vertebrae in their nominal position and fusing them in place, the relative movement of the vertebrae is eliminated.
Another approach involves the use of spinal devices to stabilize and limit, but not necessarily eliminate, the relative movement of adjacent vertebra. Such devices may include bars, rods, plates, springs, or combinations thereof connecting two sides of a vertebra, the adjacent vertebra of a motion segment unit or both. Although these devices may preserve some mobility of the motion segment units, the devices still impart a certain amount of rigidity to the spine.
A significant problem associated with either approach, but especially spinal fusion, involves the accelerated degeneration of vertebrae and vertebral discs neighboring the stabilized and/or fused motion segment unit. As described above, spinal fusion and stabilization either eliminates or reduces the mobility of one or more motion segment units. As a result, vertebrae and intervertebral discs neighboring the fused or stabilized motion segment unit must accommodate an even greater degree of motion. This added stress can lead to degeneration of the neighboring vertebrae and intervertebral discs. However, stabilization of these vertebrae may delay or eliminate such degeneration. However, stabilization of these neighboring segments may delay or eliminate such degeneration.